The present invention relates to a telephone exchange apparatus, and more particularly, to a telephone exchange apparatus having an ISDN (Integrated Services Digital Network) interface.
Generally, exclusive telephones dedicated to a system comprise a display unit for displaying an outline connecting status, however, telephone used as an extension ISDN terminal does not have such display unit.
For this reason, when a speaker at a dedicated exclusive telephone wishes to transfer a call from an outside line or an extension to an extension ISDN terminal, the speaker holds the call and inputs an extension number of the terminal so that the call is transferred to the terminal.
Accordingly, even if the terminal is near the transferring extension, the speaker of the extension should hold the call and inputs the extension number of the terminal.
Further, in case of an incoming call from an outside line, only the telephones which have been designated to ring at incoming calls by setting a PBX (Private Branch Exchange) in advance can respond to the incoming call.
In this case, a problem is that a user at a terminal which is not set to response to incoming calls cannot answer the incoming call. If all the extension ISDN terminals are set to be responsive, they ring at the incoming call at once, annoying users.
Generally, in order to connect an extension ISDN terminal to an outside line, a number "0" is supplied to the PBX before a destination dialing number. Upon reception of "0" from the extension ISDN terminal, the PBX determines that the call is connected to an outside line and searches a plurality of connected outside lines for a non-busy outside line in a predetermined order (e.g., from outside line No. 1). Thereafter, the PBX supplies the dialing signal to the seized outside line.
If a plurality of outside lines of different networks are connected to the system, an undesired outside line might be selected or even an erroneous dialing signal might be generated, confusing the operation.
Further, extension ISDN terminals connected to a PBX having an extension ISDN interface do not have a call-pickup function for responding in place of a called extension. For this reason, if there is an extension call at one of the terminals from another terminal and nobody is at the called terminal, a person who has noticed the call has to go there to answer, which is troublesome to the users.